1. Field of the Invention
The present invention relates generally to the field of energizing high intensity discharge lamps of the types having irregular characteristics which require ballast elements to control their power levels, and more particularly to closed loop control circuitry for energizing these lamps under a variety of lamp operating conditions. Specifically, the invention discloses apparatus for energizing these lamp loads using very useful attributes of certain filter networks operated under closely controlled conditions as an intrinsic part of the control techniques disclosed.
2. The Prior Art
The use of ballast elements of many types as integral portions of discharge lamp energizing circuits are, of course, well known in the electrical powering arts. A wide variety of methods and apparatus have evolved over the years to address the particular challenges offered by gas discharge lamps due to the irregular and variable volt/amp characteristics. Because the light emitting benefits offered by these lamps are significant, circuit designers have been impelled to accommodate drive circuitry to the lamp characteristics despite the inconstancy of these characteristics as the lamps have been improved and changed over the years.
High intensity gas discharge lamps, such as mercury vapor or sodium vapor types, particularly exhibit these very irregular characteristics, and as higher and higher lamp power levels are achieved, energizing methods and apparatus have had to become more astute. This is especially true due to the higher potentially destructive voltages and currents encountered in these recent lamp/power supply systems.
Prior art devices have long been directed to the disparate tasks of initially igniting these lamps (using voltages many times higher than their rated voltages); thereafter maintaining constant power levels for normal operating conditions (using more energy efficient coupling networks); and finally restarting hot or cold lamps whose plasma may have extinguished (using in many cases complex ignition and/or re-ignition approaches). For each of these operating phases, prior art designs have proposed to achieve some of the above desirable control features while minimizing heating and overloading effects, and avoiding destructively high transient voltages and currents.
Early prior art devices and circuits which addressed one or more of these varied requirements for energizing gas discharge lamps outlined above may be found in U.S. Pat. Nos. 2,659,037 to Claude and 3,519,881 to Engel et al. In the 1953 Claude patent, a form of resonant LC network operating at the power line frequency (220 VAC, 50 Hz is disclosed for operating an argon/mercury lamp, and uses a relay to switch additional reactances into the lamp powering circuit once the starting or ignition of the lamp has been achieved.
The 1970 Engel et al. patent also discloses a discharge lamp starting and operating circuit including ballast elements operated at line frequency (60 Hz), but one which generates a complex waveform having within each half cycle a very short high voltage, low energy ignition pulse a high energy starting pulse at twice line voltage needed for some lamps, and an extended normal voltage continuation.
More contemporary discharge lamp energizing approaches increasingly employ switching inverter power sources enabling them to exercise more design control over various aspects of circuit/load conditions. In this latter group are U.S. Pat. Nos. 4,277,728 to Stevens, 4,060,752 to Walker, and 3,611,021 to Wallace.
Both the Stevens and Wallace patents rely on controlling the operating frequency of switching type power inverters which apply their square wave outputs via inductor/resonant networks to high intensity discharge lamp loads. In the Walker patent, feedback signals are used to increase the inverter oscillator frequency with increasing load impedence to provide constant output power. The Stevens patent teaches the use of quadrature phase detection circuitry to control the switching frequency so that the inverter feeds a resistive load through a resonant network.
In U.S. Pat. No. 4,748,551, Dickey describes a circuit useful for maintaining a constant flow of current in spite of variations in the resistance of a load. The circuit includes a transmission line having a delay equal to an odd number of quarter cycles of a fundamental.
Therefore, it is clear that while considerable effort has been directed over the years to the problems associated with energizing gas discharge lamps, there continues to be a need for more capable circuitry that more adroitly meets the needs of recently available higher powered lamps, and that incorporates recently developed solid state circuitry for its implementation. The adaptive resonant control circuits taught in the present invention admirably meet these needs, and provide a significant improvement to the art of energizing discharge lamp loads of markedly irregular characteristics.